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An athletic conference is a collection of sports teams, playing competitively against each other at the professional, collegiate, or high school level. In many cases conferences are subdivided into smaller and smaller divisions, with the best teams competing at successively higher levels. Conferences often, but not always, include teams from a common geographic region.
United States 
In the United States and Canada, the National Hockey League (NHL) and National Basketball Association (NBA) are divided into the western (NHL, NBA) and eastern (NHL, NBA) conferences, with three divisions within each conference. In both leagues, the winners of the three divisions in each conference automatically advance to the playoffs, with the 5 next best teams in each conference also going to the playoffs. In the NHL each of the three division winner is guaranteed one of the top three seeds (or ranks to determine who has home advantage and the matchups for each round of the playoffs, where the top ranked teams play lower ranked teams) in the NBA the top four seeds go to the three division winners and the next best team, based on record. A pending realignment will divide the NHL into four regional conferences instead of two.
The National Football League has an American Football Conference (AFC) and a National Football Conference (NFC). Both conferences have 16 teams, and each conference is divided into 4 divisions of 4 teams each. Each of the 4 division winners is guaranteed one of the top 4 seeds in the playoffs. These conferences, for the most part, derive from the fact that they were once separate organizations: the original National Football League and the 1960s American Football League; the two entities merged in 1970, with each league forming the basis of the NFC and AFC respectively.
Major League Baseball does not use the word "conference." Instead, it is divided into two separate leagues which are identical to the conferences listed above in all but name(which, although their operations have been integrated via the Commissioner of Baseball in modern times, were originally separately managed organizations with an intense rivalry); These are the American League and National League with 15 teams each. Each league is divided into the Eastern, Central and Western divisions, with all six divisions having 5 teams. Each division winner is also guaranteed one of the top three seeds, even if their record is lower than the league's top wildcard team.
In all four sports, the champion of one conference (or league in MLB's case) plays the champion of the other conference for the final round championship, this is guaranteed to occur because the rules for the playoffs require play to be exclusively within the conference/league in all rounds before the final round, leaving only two teams for the finals (one from each conference/league) and the records of teams outside a conference/league are ignored, which can allow teams with inferior records to make the playoffs while teams in the other conference with better records do not get in. An extreme example of this occurred in the NFL in 2010 in regards to the Seattle Seahawks, who achieved a playoff position despite having a losing record (the only time this has happened in a non-strike season). Also, in each sport, teams play predominately within their own conference during the regular season, but play some games outside their conference (or league in baseball).
In college sports, the terms "league," "conference" and (generally at lower levels) "athletic association" can be used interchangeably to refer to a group (usually of approximately ten colleges and/or universities) of teams that regularly play against each other within a national governing body, the most significant of which is the National Collegiate Athletic Association (NCAA). Most of these groups (including all of those in the Bowl Championship Series) refer to themselves as conferences, although the Horizon League, Ivy League, Patriot League, Pioneer League and Summit League use the word "league" instead, and another conference calls itself the Colonial Athletic Association. The NCAA itself is divided into divisions and subdivisions, which can lead to redundancy when these conferences also have divisions of their own. For instance, the Southeastern Conference is part of NCAA Division I (specifically the Football Bowl Subdivision), but is itself also subdivided into an east and west division.
England and Wales 
In English Association football, Conference National is one level below the The Football League. The second tier of which is divided into Conference North and Conference South. Although, this does not meet the above definition as Conference National is the fifth tier of the English football pyramid whilst Conferences North and South are the sixth.
Australia, New Zealand and South Africa 
In Australia, New Zealand and South Africa, the highest level of provincial rugby union is Super Rugby. From the 2011 season, Super Rugby will work on a conference system. The competition will have one conference based in each country. The winner of each conference will receive a play-off spot, as will the three next best teams overall.